u/Itchy-Tomatillo3928

By the title i mean for example, the current program i am following tells me to do 1 set of bicep curls, then move onto 1 set of tricep extensions, but I prefer doing 3 sets of bicep curls, and then move onto a different exercise. Can I change it up like that, or does the order matter?

I've been lifting 3x a week for a month now and but recently whenever I do something, lets take bicep curls for example, i literally don't have enough energy to lift my arm up, but my muscle still feels like it can do a lot more reps. This only started to happen around last week. Is it because of improper form?